This invention relates to improvements in or relating to electronic gas blenders in general and is particular to electronic gas blenders for medical applications such as surgical operations, ventilation, and the like, and gas flow control mechanisms therefor.
Traditionally, in medical applications, the flows of oxygen, air and nitrous oxide are manually controlled by means of calibrated rotameters with their associated needle valves.
During surgical operations, the anesthesiologist sets the flow of each gas to deliver an appropriate total flow with the desired oxygen concentration. The oxygen concentration is usually monitored by means of an oxygen analizer which contains a means for alerting the operator of any dangerously inappropriate conditions. The manual needle valves provide complete control and the rotameters provide visual affirmation of the flow of each gas.
There are several disadvantages to this traditional delivery of gases during surgical operations. To administer the appropriate flow of gases with the desired oxygen and nitrous oxide concentration, the operator has to estimate the flow of each gas manually. If the total flow needs to be changed, each gas flow must be manually re-adjusted. This requires a great deal of mental effort and may be stressful in rapidly changing situations.
An improved method of mixing gases for medical applications utilizes a mechanical blender, like that manufactured by the Bird company (U.S. Pat. No. 3,895,642). Although mechanical blenders are simple and reliable, they have limited accuracy, especially at low flow rates. They are designed to blend two specific gases only, usually air and 02 or N20 and 02 and therefore a separate blender is needed for each combination of gases. Being mechanical, they also lack any indicators of diminished accuracy and any possibility of automatic documentation of gas flows and/or concentrations.
U.S. Pat. No. 5,887,611 discloses a microprocessor controlled gas-blender, the disclosure of which is incorporated herein by reference. It uses an 02 sensor in a feedback loop to control the gas flows. A major drawback of this device is that it depends entirely on the accuracy of the 02 sensor for its operation. If the 02 sensor fails or becomes inaccurate, the entire system may fail or become inaccurate. Another inherent drawback of this device being microprocessor controlled is the limited resolution of the flow control. A limiting characteristic of microprocessor (digital) operation is that the resolution of flow control is discontinuous. It can only provide a number of discrete steps within its range of control, where the number of steps depends on the precision of the analog to digital conversion. The resolution limitations may also preclude the use of such a blender at lower flows, because its accuracy would be severely degraded.
A more versatile device used for controlling the flow of gases is the Mass Flow Controller (MFC). It is highly accurate, but suffers from several disadvantages. A problem with the MFC is its slow response. Even a faster MFC may require a response time of over a second. This slow response precludes the MFC from use in dynamic situations where a fast response is crucial. In closed-loop feedback applications, the slow response of the MFC causes undesirable effects like “popping” on turn-on (overshoot) and very sluggish response to changes in flow setting. It is also bulky, heavy and relatively expensive.